if you take a sword or any other none enchanted item and than click it in and out of the enchanting box very quickly it will duplicate the sword. (the duplicate will not last very long but you still can use it a little, some times)(works best on servers)
MC-50110
Enchanting
Resolved
Invalid
0
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]) here.